The present invention relates to an amplification-type solid image pick-up unit and a method of driving the same, and relates more particularly to an amplification-type solid image pick-up unit which can improve an S/N ratio and which can adjust sensitivity easily and a method of driving this solid image pick-up unit.
As an example of the MOS-type image sensor that has been used conventionally, there has been known a one having a structure as described in the Japanese Patent Application No. 8-275949 which is a prior application made by the inventor of the present invention.
FIG. 1 a diagram for showing a circuit structure of the main portion of the image sensor described in the Japanese Patent Application No. 8-275949.
In FIG. 1, this image sensor has a photodiode 2 connected to a reset transistor 6 and an amplification transistor 8 through a transfer transistor 4. A circuit structured by a transfer transistor 10a and sample hold capacitor 12a and a circuit structured by a transfer transistor 10b and sample hold capacitor 12b are connected in parallel respectively between the transfer transistor 4 and the amplification transistor 8.
The amplification transistor 8 has its drain connected to the source of an address transistor 14 and has its source connected to a vertical signal line 16. The gate of the address transistor 14 is connected to a horizontal address line 18. Further, the amplification transistor 8 is connected to a horizontal selection transistor through the vertical signal line 16 and a differential circuit not shown.
In the above-described structure, it becomes possible to cancel a variation in the threshold value of the amplification transistor 8 by taking a difference between the potential of the vertical signal line 16 when the gate of the amplification transistor 8 has been reset and the potential of the vertical signal line 16 when a signal component is being applied.
According to the image sensor of the structure as shown in FIG. 1, it is not so arranged that a signal charge obtained from the photodiode 2 is taken out directly. However, it is so arranged that the simultaneity of the signal charges obtained from the photodiode 2 is obtained by providing the capacity for holding signal charges from the photodiode 12, that is the sample hold capacitor 12a and 12b. 
In the image sensor shown in FIG. 1, a light emitting diode (LED) not shown is used for deciding a light applied from the outside to an object of which image is to be picked up. In other words, a signal of only a background light such as a sun beam or an indoor light other than the light emitted by the LED and a signal of a light which is a combination of the background light and the light emitted by the LED are photoelectrically converted by the photodiode 2 and are then taken into the sample hold capacitors 12a and 12b. Then, a difference between the signal charges taken into these sample hold capacitors 12a and 12b is taken. However, when the LED is used, there arises a problem that the image sensor has a leakage of charges easily.
Although it is possible to cancel the leakage to some extent by taking a difference in the case of an external light, leaking charges depend on the distribution of the surrounding light and are not uniform on the plane. Therefore, there is a problem that there arises a difference between the quantity of leakage at a first time of storage after the LED has emitted a light and the quantity of leakage at a second time of storage after the LED has not emitted a light, for example, depending on the layout of the diffusion layer area of the transistors connected to the sample hold capacitors for holding the signal charges. Furthermore, the S/N ratio becomes poorer because of the variation in the leakage quantity.
Further, according to the above-described Japanese Patent Application No. 8-275949, since light emitting time and light emitting intervals are made shorter in order to restrict variation of the external lights, the light emitting volume becomes smaller if the number of LED cannot be increased for cost reason. To avoid this difficulty, it may be considered suitable to increase the LED light emitting time to a double. However, this often leads to a doubling of the dark current and the leaking quantity although the total light emitting volume of the LED is not doubled because of the characteristics of the LED. Accordingly, this does not contribute to improve the S/N ratio although the signal volume is increased.
On the other hand, when the number of LED is increased, the number of photon emitted is increased. However, if the number of LED is increased, it becomes impossible to reduce the size of the unit. Moreover, a larger power source is required because a large volume of current becomes necessary at one time. Therefore, a method which makes it possible to increase the sensitivity of the sensor without increasing the number of LED or without increasing the light emitting time has been required.
Further, according to the method described in the mode of implementation in the Japanese Patent Application No. 8-275949, the photodiode is saturated by the dark current the strong external light signal component at the time of adjusting the sensitivity. In order to use the unit within the range in which the photodiode is not saturated, the sensitivity adjusting operation must be repeated by many cycles. However, this has a problem of extremely limiting the number of signal storage operation because of the characteristics of the capacitors.
In order to eliminate the above-described problems, it is an object of the present invention to provide a solid image pick-up unit which can restrict the variation in the leakage charge quantity at the light emitting time and non-light emitting time of the LED, which makes it possible to increase the signal charge quantity without increasing the number of LED and which can carry out sensitivity adjustment easily, and a method of driving this unit.
According to the present invention, there is provided a solid image pick-up unit having a unit cell laid out in a matrix two dimensional shape, by having a light receiving unit, an amplifying unit for amplifying a signal detected by the light receiving unit, a selecting unit for selecting a cell thereof and a resetting unit for resetting the light receiving unit, on a semiconductor substrate, wherein the unit cell has at least two storage units for storing a signal corresponding to a light quantity received by the light receiving unit, the semiconductor substrate is structured by a first conductive type and has a difference detecting unit for detecting a difference between signals stored in the at least two storage units, the light receiving unit is structured by a first diffusion layer area of a second conductive type opposite to the first conductive type, the at least two storage units are structured by a second diffusion layer area of a second conductive type, and a first diffusion layer area of the storage units is disposed at a position symmetrical with the center of the first diffusion layer area of the light receiving unit.
Further, according to the present invention, there is provided a solid image pick-up unit having a unit cell laid out in a matrix two dimensional shape, by having a light receiving unit, a detecting unit for detecting a signal charge by transferring a signal from the light receiving unit, an amplifying unit for amplifying a signal detected by the detecting unit, a selecting unit for selecting a cell thereof and a resetting unit for resetting the light receiving unit, on a semiconductor substrate, wherein the detecting unit is structured by first and second detecting units connected in series, the solid image pick-up unit includes a storage unit connected between the first detecting unit and the second detecting unit, and the resetting unit also resets the amplifying unit and the storage unit.
Further, according to the present invention, there is provided a solid image pick-up unit having a unit cell laid out in a matrix two dimensional shape, by having a photodiode, a transfer transistor for transferring a signal from the photodiode, a detection transistor for detecting a signal charge transferred by the transfer transistor, an amplification transistor for amplifying a signal detected by the detection transistor, a selection transistor for selecting a cell thereof, a reset transistor for resetting the photodiode and a sample hold capacitor for holding a signal corresponding to a light quantity received by the photodiode, on a semiconductor substrate, wherein the solid image pick-up unit includes a slice transistor to be connected to the photodiode through the transfer transistor and includes a slice capacitor having one end of an electrode thereof connected to the source and drain area of the slice transistor and having the other end of the electrode connected to a pulse circuit, and the other end of the source and drain area of the slice transistor is connected to the gate of the amplification transistor through the detection transistor, one end of the sample hold capacitor is connected to a connection section between the detection transistor and the amplification transistor, and the reset transistor is connected to a connection section between the transfer transistor and the gate of the slice transistor.
Further, according to the present invention, there is provided a solid image pick-up unit having a unit cell laid out in a matrix two dimensional shape, by having a photodiode, a transfer transistor for transferring a signal from the photodiode, a detection transistor for detecting a signal charge transferred by the transfer transistor, an amplification transistor for amplifying a signal detected by the detection transistor, a selection transistor for selecting a cell thereof, a reset transistor for resetting the photodiode and a first capacitor for holding a signal corresponding to a light quantity received by the photodiode, on a semiconductor substrate, wherein the solid image pick-up unit includes a second capacitor different from the first capacitor and connected to the gate of the amplification transistor, and the first capacitor has its one end connected to the photodiode through the transfer transistor and has the other end connected to the gate of the amplification transistor through the detection transistor, and the photodiode is connected to the reset transistor.
According to the present invention, there is provided a solid image pick-up unit having a unit cell laid out in a matrix two dimensional shape, by having a light receiving unit, an amplifying unit for amplifying a signal detected by the light receiving unit, a selecting unit for selecting a cell thereof, a resetting unit for resetting the light receiving unit and at least two storage units for storing a signal corresponding to a light quantity received by the light receiving unit, on a semiconductor substrate, wherein a selection of the storage units is carried out during an operation of the light receiving unit.
Further, according to the present invention, there is provided a method for driving a solid image pick-up unit having a unit cell laid out in a matrix two dimensional shape, by having a light receiving unit, a transfer unit for transferring a signal detected by the light receiving unit, an amplifying unit for amplifying a signal transferred by the transfer unit, a selecting unit for selecting a cell thereof, a resetting unit for resetting the light receiving unit and at least two storage units for storing a signal corresponding to a light quantity received by the light receiving unit, on a semiconductor substrate, wherein the method includes a first step of turning on the transfer unit and the resetting unit and injecting a charge into the light receiving unit and the storage units, a second step of discharging a charge injected into the storage units, a third step of transferring a signal detected by the light receiving unit to the storage units by keeping the transferring unit on, and a fourth step of reading out a charge stored in the storage units.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.